In general, an ultrasound system emits pulses over a plurality of paths and converts echoes received from objects on the plurality of paths into electrical signals used to generate ultrasound data from which an ultrasound image can be displayed. The process of obtaining the raw data from which the ultrasound data is produced is typically termed xe2x80x9cscanning,xe2x80x9d xe2x80x9csweeping,xe2x80x9d or xe2x80x9csteering a beamxe2x80x9d. In most ultrasound systems, scanning is rapidly repeated using an ultrasound transducer assembly so that many images maybe acquired within a second of time.
Ultrasound transducer assemblies comprise transducer elements, typically with damping and matching materials, that when excited by an electrical pulse emit ultrasound pulses and receive echoes. Transducer assemblies are packaged with associated electronics and connections in a housing that facilitates examination. Taken as a whole, such a combination (the transducer assembly, electronics, connections, and housing) is typically referred to as an ultrasound probe (or simply just xe2x80x9cprobexe2x80x9d). In general, modern probes are classified as either 1-D probes (having a single element wide array) or 2-D probes (having a multi-dimensional array of elements). Other types of probes do exist, such as bi-plane probes (having two 1-D arrays for scanning on two different planes).
Ultrasound data is typically acquired in frames, each frame representing a sweep of an ultrasound beam emanating from the face of a transducer. 1-D transducers produce 2-D rectangular or pie-shaped sweeps, each sweep being represented by a series of xe2x80x9clinesxe2x80x9d of data points. Each of the data points are, in effect, a value representing the intensity of an ultrasound reflection at a certain depth along a given line. Newer 2-D transducers are capable of producing sweeps forming a set of data points describing pre-defined 3-D shapes (typically referred to as a xe2x80x9cvolume scanxe2x80x9d).
Real-time sonography refers to the presentation of ultrasound images in a rapid sequential format as the scanning is being performed. Scanning is either performed mechanically (by physically oscillating one or more transducer elements) or electronically. By far, the most common type of scanning in modern ultrasound systems is electronic wherein a group of transducer elements (termed an xe2x80x9carrayxe2x80x9d) arranged in a line are excited by a set of electrical pulses, one pulse per element, timed to construct a sweeping action.
In a linear sequenced array, an aperture is swept across the array by sequentially exciting overlapping sub-groups of transducer elements. In a linear phased array, all (or almost all) the elements are excited by a single pulse, but with small (typically less than 1 microsecond) time differences (xe2x80x9cphasingxe2x80x9d) between adjacent elements, so that the resulting sound pulses pile up along a specific direction (termed xe2x80x9csteeringxe2x80x9d). In addition to steering the beam, the phased array can focus the beam, along the depth direction, by putting curvature in the phase delay pattern. More curvature places the focus closer to the transducer array, while less curvature moves the focus deeper. Delay can also be used with a linear sequenced array to provide focusing. Conversely, upon reception of echoes, delays are used to time the sampling of the raw points of data from which ultrasound image data is produced.
The apparatus that creates the various delays is called a beamformer. Known beamformers have traditionally operated in the analog domain employing expensive circuits capable of delivering a new point of data (dynamically delayed) every nano-second. More recently, digital beamformers have been developed, that provide delay by buffering A/D converted transducer output in a digital memory and varying the read times therefrom. Known digital beamformers are capable of delivering a new point of data at least every 25 nano-seconds.
To produce 3-D images a volume of ultrasound data (a 3-D scan data set) must be created, either by scanning or scanning with interpolation. This volume of data is then processed to create an image for display on a 2-D surface that has the appearance of being 3-D. Such processing is typically referred to as a rendering.
One method to generate real-time 3-D scan data sets is to perform multiple sweeps wherein each sweep oriented to a different scan plane. The scan lines of every sweep are typically arrayed across the probe""s xe2x80x9clateralxe2x80x9d dimension. The planes of the successive sweeps in a frame are rotated with respect to each other, e.g. displaced in the xe2x80x9celevationxe2x80x9d direction, which is typically orthogonal to the lateral dimension. Alternatively, successive sweeps may be rotated about a center line of the lateral dimension. In general, each scan frame comprises a plurality of lines allowing the interrogation of a 3-D scan data set representing a scan volume of some pre-determined shape, such as a cube, frustum, or cylinder.
While some 3-D optimized ultrasound systems are available, most commercial ultrasound systems today display only planar 2-D images, acquiring scan data from one-dimensional array probes. The SONOS 5500 sold by AGILENT TECHNOLOGIES, Inc. is one example of one such system. Some commercial systems, including the SONOS 5500, can generate 3-D ultrasound images with the help of xe2x80x9coff-linexe2x80x9d post-processing. To do this, sequences of regularly spaced planar 2-D sweeps are collected as the position of the probe is translated or rotated in some way between scan frames. Post-processing manipulation reconstructs 3-D data sets using acquired position information for each 2-D scan plane. The resulting 3-D data sets are displayed as rendered images, typically on a separate workstation, using any of various well-known, computation-intensive rendering techniques. Furthermore, the real-time rendering and display workstation may be integrated with the ultrasound scanner into one system, for example VOLUMETRICS, Inc. produces such a system.
One enabling technologies for real-time 3-D is the development of probes having a transducer assembly comprising a matrix of elements (for example a 56xc3x9756 array of 3,136 elements), sometimes referred to a 2-D probe. Because 2-D probes allow beam steering in two dimensions as well as the aforementioned focus in the depth direction, there is no need to physically move the probe to translate focus for the capture of a volume of ultrasound data to be used to render 3-D images.
When a series of scan frames are acquired, each containing multiple scan planes, the resulting 3-D data sets may include another dimension, time, indexed by relative time or frame number. Such time or frame indexed data sets are referred to as xe2x80x9c4-D data sets,xe2x80x9d and can be used to produce moving 3-D images on the offline workstation. If the full time-sequence of 3-D images derived from the 4-D data sets are pre-calculated for a given set of viewing parameters (viewing angle, opacity, perspective, etc.), then the moving 3-D image display can be rendered at the xe2x80x9clivexe2x80x9d frame rate, that is, the rate at which the scan frames were acquired. Given a probe whose scan beams can be electronically steered/focused in three dimensions (e.g., employing a 2-dimensional element array), and a sufficiently potent rendering system, 4-D data sets may be acquired and rendered not only at the xe2x80x9clivexe2x80x9d frame rate but also in real-time, where the acquisition, rendering, and display are done simultaneously.
Currently, 3-D systems occupy a relatively small market niche. It has proven difficult to convince users to replace their existing 2-D systems, most of which are quite expensive, with an even more costly 3-D based system that sometimes sacrifices resolution and clarity for the illusion of depth. Accordingly, the Inventor has identified a need to develop a 3-D capable ultrasound imaging system based on current commercial ultrasound systems that doesn""t require the addition of significant hardware, such as a 3-D workstation. Such a solution could be supplied to the user as a low cost upgrade to a current 2-D system.